Nuclear Magnetic Resonance
& Electron Paramagnetic Resonance
Metrolab is primarily known for its precisionPrecision is how closely multiple measurements will be clustered. Also called reproducibility or repeatability. In everyday speech, often confused with... More NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More teslameters. These can also use EPRElectron Paramagnetic Resonance – same as ESR. More probes to measure low magnetic fields. In this section, we want to tell you a little more about these – our – technologies.
How NMR magnetometers work
15 minutes
Advantages
Reads total field
Extremely precise
Constraints
Low-field NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More requires large sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More size (alternative is ESRElectron Spin Resonance. Effect similar to Nuclear Magnetic Resonance, except we're manipulating the spin of the electron rather than the... More)
A quantum effect
If a nucleus has spinElectrons and protons have spin; in other words, they act as if they were spinning on their axis. This means... More, it tends to align itself to an external magnetic field. However, by giving it exactly the right additional amount of energy, the nucleus can be induced to flip into the opposite spinElectrons and protons have spin; in other words, they act as if they were spinning on their axis. This means... More state. Nuclear Magnetic Resonance (NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More) occurs when a radio-frequency field applied to a sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More is just the right frequency – called the Larmor frequencyThe frequency at which an NMR sample resonates. Depends linearly on the applied external field. More – to induce this spin-flip. Electron Paramagnetic Resonance (EPRElectron Paramagnetic Resonance – same as ESR. More) is a similar effect, with an electron rather than a nucleus.
Gyromagnetic ratios
It turns out that the energy difference between the aligned and counter-aligned nuclear states depends linearly on the field strength. Thus the ratio of the resonant frequency to field strength is a physical constant, called the gyromagnetic ratioIn NMR or ESR, the ratio of the resonant frequency to magnetic flux density. Varies from one nucleus to the... More (gamma). It is approximately 42.5 MHz/T for protons (hydrogen nuclei).
Other nuclei also exhibit NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More, but with a different gamma, for example, 6.5 MHz/T for deuterium and 40 MHz/T for fluorine. EPRElectron Paramagnetic Resonance – same as ESR. More has a much higher gamma, approximately 28 GHz/T.
The perfect magnetometer
The heart of an NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More magnetometerAn instrument to measure magnetic flux density (B) or magnetic field intensity (H). More is simply a coil wrapped around a sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More material. The coil provides the RF energy needed to induce spinElectrons and protons have spin; in other words, they act as if they were spinning on their axis. This means... More flips in the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More. Setting the axis of the coil perpendicular to the field enables the acquisition of the best response. The resolutionResolution measures the ability of a magnetometer to distinguish ("resolve") two nearly identical field values. Related to precision, but not to... More is limited only by the resonance width, which, depending on the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More material, can be very narrow indeed, on the order of 1 Hz.
Besides, NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More always measures total field strength, rather than a single componentThe magnetic field is a three-dimensional vector quantity, with three components. Magnetometers may measure 1, 2 or 3 components, or the vector magnitude. More. Last but not least, since the gyromagnetic ratioIn NMR or ESR, the ratio of the resonant frequency to magnetic flux density. Varies from one nucleus to the... More is a physical constant, the NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More magnetometerAn instrument to measure magnetic flux density (B) or magnetic field intensity (H). More has practically no driftThe gradual loss of an instrument's accuracy. NMR teslameters drift because their time base drifts; this can be easily checked and... More and requires no calibrationDuring calibration, the instrument reading is compared to one or more references to verify its accuracy. The references themselves need... More.
Continuous- versus pulsed-wave
There are two fundamental methods of detecting Nuclear Magnetic Resonance. The continuous-waveIn NMR magnetometry, continuous-wave is one approach to finding the NMR resonance frequency, using a continuous excitation procedure. The other... More approach is like tuningNMR teslameter probes usually have a capacitor placed in parallel with the RF coil, forming an LC resonator that increases... More a radio: we slowly adjust the frequency until we “tune in” the resonance. To be able to detect it, we must cross and re-cross the resonance frequency, which means we must modulate either the frequency or the magnetic field.
The pulsed-wave approach, on the other hand, is like ringing a bell: we strike the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More with a broad-band pulse, and the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More absorbs and reradiates at the Larmor frequencyThe frequency at which an NMR sample resonates. Depends linearly on the applied external field. More. The pulsed mode approach requires modern, fast-switching electronics, but it is more straightforward and generally delivers greater precisionPrecision is how closely multiple measurements will be clustered. Also called reproducibility or repeatability. In everyday speech, often confused with... More.
Limitations: measurement rate
After a resonance, the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More is usually allowed to shed the absorbed energy and regain its original, spin-aligned state. This limits the measurement rate to around 10-100 Hz. Also, to reduce the variability, many such measurements are often averaged together, yielding effective measurement rates approaching a second. Therefore, NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More is usually only useful for slowly changing fields.
Field uniformity
If the field is not uniformSee homogeneous. More, one edge of the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More will resonate at a different frequency from the other side. The resonance peak broadens and flattens until it completely disappears in the noise. This phenomenon determines the field homogeneity limit of NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More.
Measurement range
There are also practical limitations to the useful rangeThe range of a probe is defined by the minimum and maximum field strength it can measure. On an instrument,... More of NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More. For low fields, the nuclei are only weakly aligned, and the NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More response fades. The simplest solution is to use a larger sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More, thereby increasing the number of participating spins; however, at a certain point, the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More size becomes impractically large for a general-purpose probeThe actual sensor that is placed in the magnetic field. The NMR probe contains the NMR sample; the Hall probe... More.
For high fields, the NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More response is excellent, but the instrument’s RF generator and the coil inductance impose an upper bound on attainable frequencies. To diminish inductance, the size and number of whorls in the coil are reduced. A practical limit is reached when the coil is reduced to a single tiny loop.
Tuning
The coil around the sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More is an inductor, with a response that rolls off at high frequencies. If we add capacitance in parallel, we create an LC resonator, which, if tuned to coincide with the Larmor frequencyThe frequency at which an NMR sample resonates. Depends linearly on the applied external field. More, dramatically improves the sensitivity of the NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More signal detection. For this reason, NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More probes require tuningNMR teslameter probes usually have a capacitor placed in parallel with the RF coil, forming an LC resonator that increases... More.
At Metrolab, we tune our NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More probes with a fixed capacitor or trim cap. In this case, the probeThe actual sensor that is placed in the magnetic field. The NMR probe contains the NMR sample; the Hall probe... More is manually tuned for a given frequency – as is the case, for example, for Metrolab’s Magnetic Field Camera. For our PrecisionPrecision is how closely multiple measurements will be clustered. Also called reproducibility or repeatability. In everyday speech, often confused with... More TeslameterSee magnetometer. More, we want a single probeThe actual sensor that is placed in the magnetic field. The NMR probe contains the NMR sample; the Hall probe... More to cover as broad a rangeThe range of a probe is defined by the minimum and maximum field strength it can measure. On an instrument,... More as possible, and, in that case, we tune with an electronically controlled varicap. The dynamic rangeThe range of a probe is defined by the minimum and maximum field strength it can measure. On an instrument,... More of the varicap imposes a practical limit to the measurement rangeThe range of a probe is defined by the minimum and maximum field strength it can measure. On an instrument,... More of a single probeThe actual sensor that is placed in the magnetic field. The NMR probe contains the NMR sample; the Hall probe... More.
Changing sample materials
To extend the measurement rangeThe range of a probe is defined by the minimum and maximum field strength it can measure. On an instrument,... More of NMRNuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field.... More, we can use a sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More with a different gamma. For example, a deuterium sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More has a gamma that is a factor 6.5 lower than the proton’s, so the same frequency rangeThe range of a probe is defined by the minimum and maximum field strength it can measure. On an instrument,... More will measure fields 6.5x higher. Conversely, an ESRElectron Spin Resonance. Effect similar to Nuclear Magnetic Resonance, except we're manipulating the spin of the electron rather than the... More sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More, with a gamma 2.5 orders of magnitudeThe magnetic field is a vector quantity. We may be interested in its individual components or its total magnitude. More higher than the proton’s, will measure magnetic fields 2.5 orders of magnitudeThe magnetic field is a vector quantity. We may be interested in its individual components or its total magnitude. More lower. Unfortunately, both these sampleThe NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of... More materials have practical drawbacks compared to ordinary proton samples.
More technologies
Products
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PT2026 NMR Precision Teslameter
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Applications
Related applications
MRI
PrecisionPrecision is how closely multiple measurements will be clustered. Also called reproducibility or repeatability. In everyday speech, often confused with... More field mappingThe process of measuring magnetic field intensity at many different points, in order to understand the structure of the field... More
Metrology
Reference field
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